1. Field of the Invention
The present invention relates generally to energy absorbers and energy absorption systems, and more particularly, to adaptive energy attenuation systems for impact events, and control thereof.
2. Description of the Background
The primary function of a shock and vibration protection system is to minimize the potential for equipment damage and/or personnel injury during shock and vibration loading. Such systems are important for vehicular applications, including aircraft, ground vehicles, marine vehicles, etc. Severe shock events may include harsh vertical or crash landings of aircraft, severe water impacts of high speed watercraft, and harsh ground impacts of ground vehicles, or even horizontal crashes of ground vehicles. Moreover, lower amplitude shock and vibration tend to result from normal operation of such vehicles, including aircraft air loads or rotor loads, ground vehicles traversing rough terrain, etc. The severity of equipment damage and/or personnel injuries can be considerably minimized if the vehicles are equipped with shock and vibration protection systems. Most existing protection systems employ energy absorbers (EAs) as opposed to springs or other energy storage devices because the latter are prone to inflicting harmful force as they recoil or rebound. EA is herein defined as any suitable device used to absorb energy by providing a resistive force applied over a deformation distance without significant elastic rebound. EAs damp applied forces but do not store them to any significant degree (as do coil springs). EAs include fixed profile energy absorbers (FPEAs) which have a constant load-stroke curve, such as standard hydraulic or pneumatic cylinders. However, FPEAs are passive, meaning that they cannot automatically adapt their energy absorption as a function of payload weight or as a function of real-time environmental measurements such as shock level, impact velocity, vibration levels, etc. Moreover, some FPEAs are essentially very stiff and therefore do not stroke until the load reaches a tuned threshold. Because of this, these systems provide little to no isolation of vibration. This motivates the use of a shock and vibration protection system that utilizes an electronically adjustable adaptive energy absorber, or “variable profile energy absorber” (VPEA) that can provide adaptive energy absorption for enhanced crashworthiness as well as vibration mitigation. VPEAs impart a controlled resistive force that can be continuously adjusted over a known deformation distance of the VPEA. Since the resistive force can be continuously adjusted over the deformation distance, the VPEA can be controlled in real time to respond to changing environmental stimuli including load levels to effectively mitigate loads into the occupant's body. Suitable VPEAs may comprise any of an active valve damper, a magnetorheological fluid damper (including rotary magnetorheological fluid brake or clutch), an electrorheological fluid damper, a magnetic energy absorber, a servo-hydraulic actuator (with an orifice adjusted by electromechanical actuator), or an electronically adjustable friction device such as a piezo-electric friction damper or magnetically controlled friction damper. Active valve dampers are pneumatic or hydraulic cylinders that rely on internal valving changes to automatically adjust their damping effect. Active valve dampers with electrically controlled damping constants are known in the art, and typically use variable valve orifices to adjust the damping force.
Magnetorheological (MR) technology is particularly attractive for shock and vibration protection systems because an MR fluid based device is capable of achieving what is effectively a continuously adjustable energy absorber. An MR fluid based device in combination with a real-time feedback controller can automatically adapt to payload weight and respond to changing excitation levels. With its ability to smoothly adjust its load-stroke profile, MR energy absorbers can provide the optimum combination of short stroking distance and minimum loading while automatically adjusting for the payload weight and load level. Furthermore, MR energy absorbers offer the unique ability to use the same system for vibration isolation.
One key challenge in controlling an electronically adjustable energy absorption system is determining how to adapt the force levels to effectively minimize the loads transmitted to the payload for each individual impact. If an adaptive energy absorption system only reacts to a measured impact pulse, the system may exhaust a considerable amount of its limited stroke capability before it is able to make any force adjustments. This wasted stroke will either necessitate additional stroking capability (which in turn requires larger energy absorbers and so may not be geometrically possible for a given application), or else risk the system reaching end-stop impact (which could be harmful to the payload). Moreover, many adaptive suspension systems include a spring as a restorative element to prepare for subsequent impacts. Springs impart a return force that is proportional to stroking distance. Consequently, utilizing more stroke than necessary in a spring-return adaptive suspension system is not ideal because it can result in more force being transmitted to the payload.
The kinetic energy of a suspended payload prior to impact is given by equation [1]:
                              E          K                =                              1            2                    ⁢                      MV            p            2                                              [        1        ]            
where M is the payload mass and Vp is the absolute velocity of the payload just prior to impact. Upon impact, this energy will be converted to energy absorbed and/or stored by the impact surface (ground, floor, wall, etc.), energy absorbed (damping) and/or stored (stiffness) by any payload substructure (beneath the payload and energy absorption system), and energy absorbed and/or stored by the adaptive energy absorption system. Depending upon how “soft” or “stiff” the impact surface and payload substructure is, the resulting impact acceleration pulse will be correspondingly low or high in magnitude and duration. That is, an impact on a stiff surface with stiff or no payload substructure will result in a higher magnitude and short duration acceleration pulse than one on a soft surface with flexible or crushable substructure with the same initial kinetic energy. Fortunately, the energy absorption and energy storage properties of the impact surface and payload substructure are often common across particular applications, and so a particular application itself establishes certain parameters useful for characterizing impact events.
It would be greatly advantageous to provide an adaptive energy absorption system that determines the severity of a particular impact event a priori. The invention disclosed herein provides a means to determine such impact severity as well as strategies for controlling an energy absorption system in accordance therewith.